Internal combustion engine blocks and engine components are frequently produced by an evaporable foam casting process. In the typical evaporable foam casting process, a pattern, identical in shape to the cast part to be produced, is formed from an evaporable foam material, such as expanded polystyrene. The pattern is placed in a mold or flask, and the mold is filled with a flowable material, such as sand, which also fills the cavities of the pattern. A molten metal is introduced to the pattern via a sprue and the heat of the molten metal will vaporize the foam material with the vapor passing into the interstices of the sand, while the molten metal will fill the void created by vaporization of the foam material. The result is a cast metal part having a configuration identical to that of the foam pattern.
Evaporable foam casting processes have distinct advantages. In certain instances a number of components can be combined in the foam pattern to provide an integral cast part, as opposed to sand casting processes where the components are individually cast and then connected together by gaskets and bolts.
The use of evaporable foam patterns also increases the flexibility of the casting process and enables the pattern to be designed so as to reduce the weight of metal to be used and provide more uniform wall thicknesses to increase the rate of heat transfer.
A V-6 stern drive marine engine includes a pair of exhaust manifolds which are mounted on either side of the engine. The typical exhaust manifold consists of a generally flat upright body portion which is merged into a curved elbow portion. The manifold is composed of an inner exhaust conduit which is surrounded by an outer water jacket to define a water passage therebetween. The exhaust conduit is formed with a plurality of inlet ports which register with exhaust ports in a bank of cylinders of the engine, and the exhaust conduit terminates in a downwardly facing outlet located in the elbow portion and the outlet is positioned concentrically within the water jacket.
In the past, exhaust manifolds have been formed of two separate sections, namely the body portion and the elbow portion, which are joined together by bolts. The bolted construction requires that the mating surfaces having an increased section thickness and it is necessary to drill and tap holes in the mating surfaces to receive the bolts. In addition, gaskets are required at the mating surfaces.
The bolted connection not only increases the overall weight of the exhaust manifold due to the requirement for increased section thickness, but substantially increases the overall cost of the manifold due to the machining, drilling and tapping requirements, as well as the necessity of using gaskets and bolts.
In other instances, cast iron exhaust manifolds have been produced as a single piece by sand casting techniques, utilizing a two-piece water jacket core and an exhaust passage core. However, sand casting of the manifold requires external and internal core support systems and necessitates the use of relatively thick cores to add strength to the cores during assembly and during metal pouring. In addition, sand casting requires additional parts and operations to subsequently plug the core holes. Further, sand cast parts require thicker wall sections to enhance metal flow. Because of this, sand cast parts are physically larger castings which, when used in marine applications, infringe on usable space in the boat as well as decreasing the power to weight ratio of the boat. Moreover, sand cast parts require considerable machining which adds to the overall cost.